. >> make no mistake, president obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people. >> reporter: images from the attack are hard to watch. secretary kerry spoke about the hundreds, maybe thousands of innocents who were killed. >> as a father, i can't get the image out of my head of a man who held up his dead child, wailing while chaos swirled around him, the images of entire families dead in their beds without a drop of blood or a visible wound, bodies contorting in spasms, human suffering that we can never ignore or forget. >> reporter: such strong words from top u.s. officials leave little doubt the u.s. plans military action against syria's assad regime as three key allies, britain, france and turkey, say they will back a military response, u.n. mandate or not. assad himself remains defiant, warning that the u.s. would fail as in vietnam. diane, u.s. officials say the purpose of any military strike would be to deter and prevent syria from launching any more chemical weapons attacks which mean